1. Field of the Invention
The present invention is directed to the field of radiation shielding for radioactive materials. More particularly, the present invention is directed to a process for producing an aggregate suitable for inclusion into a radiation shielding material.
2. Relevant Technology
With the continuous increase in the amounts of various radioactive materials generated from nuclear power plants and other nuclear facilities, substantial efforts are being made to develop safe and economical ways to store and dispose of radioactive materials. A significant portion of the efforts that have been made are directed to radiation shielding having improved radiation shielding compositions for containers, containment systems and the like, wherein the radioactive materials are contained over extended periods of time.
Radioactive materials, including liquids from reprocessing and spent nuclear fuel, typically have half-lives of hundreds of thousands of years. These radioactive materials are generally stored as a liquid, then solidified, permanently stored, and disposed of as required. For instance, spent nuclear fuel is initially stored in water-cooled pools at reactor sites and subsequently moved to dry storage containers until such time that permanent disposal facilities become available.
In order to protect the human body, machines, and tools from the harmful influences of radiation from radioactive materials, radiation shielding means are prepared having both gamma radiation and neutron shielding characteristics. One material commonly used in radiation shielding is concrete. Concrete is widely used because of its solid structure, low cost and workability.
Shielding efficiency increases as the specific gravity of the radiation shielding means increases. Hence, concrete having a high specific gravity is preferred. However, radiation shielding formed from concrete alone must be extremely thick to provide adequate shielding for radioactive materials. In fact, concrete radiation shielding systems require such a great thickness that these systems generally lack portability due to their high mass and substantial bulk. In addition, this thickness limits the volume of the radioactive material that can be stored in the system because of the space required.
Yoshihisa et al., Japanese Patent Application Pub. No. 61-091598, teaches that shielding characteristics of radiation shielding formed from concrete can be improved by adding depleted uranium metal or uranium oxides to concrete mixtures. Unfortunately, efforts to utilize depleted uranium compounds have been largely unsuccessful, due in part to the chemical reactivity of many of the depleted uranium compounds. For example, the mixed depleted uranium or uranium oxide compounds frequently undergo reactions in the concrete that result in the degradation of the concrete, which may prevent the concrete mixture from obtaining the desired system life of one hundred years, particularly at elevated temperatures.
For depleted uranium to be useful as a suitable aggregate for radiation shielding, the depleted uranium aggregate must not be chemically reactive to its environment. Furthermore, the depleted uranium must have a chemical composition optimized with respect to density, microstructure (fine grained with a minimum of large porosity), leach resistance, and neutron and gamma ray attenuation. Likewise, it is equally important that the uranium aggregate be produced at a low enough cost to allow the concrete mixture containing the uranium aggregate to be viable for disposal or to fabricate shielding structures such as storage casks for spent nuclear fuel.
More recently, in an attempt to produce a stable depleted uranium aggregate having the desired parameters at a low cost, researchers at the Idaho National Engineering & Environmental Laboratory (INEEL) have developed methods for producing high-density aggregates for concrete primarily consisting of depleted uranium oxide. The INEEL method takes a finely divided powder consisting of uranium oxide and increases its packed density by pressing, followed by liquid phase sintering at elevated temperatures. The dense stabilized uranium oxide aggregate is then added to concrete to form a concrete mixture having a higher density to be used for radiation shielding purposes. The cost of fabricating spent nuclear fuel storage casks for uranium oxide aggregates is comparable to those for conventional concrete when using an estimate of $2.20/kg-U ($1.00/lb-U) for converting powdered UO.sub.3 or U.sub.3 O.sub.8 into dense UO.sub.2 aggregate and $0.12/kg-U for fabricating the concrete containing the uranium oxide aggregate into a cylindrical storage cask. Hence, this process for forming useful aggregates from uranium oxides is very useful for the efficient disposal of uranium oxide. A problem, however, is that the Department of Energy currently has about 555,000 metric tons of uranium hexafluoride in need of disposal. In comparison, the Department of Energy currently has only about 20,000 metric tons of uranium oxide in need of disposal. Presently, there is no known process for economically converting uranium hexafluoride directly into a stable aggregate for addition to concrete radiation shielding.
One possibility of disposing of the uranium hexafluoride is to convert the uranium hexafluoride into uranium oxide and then form a dense uranium oxide aggregate as referred to in the INEEL method above. However, to use the presently used process of forming dense uranium oxide aggregates for addition to concrete mixtures requires the depleted uranium hexafluoride to be first converted to a uranium oxide. The expense of converting the uranium hexafluoride into uranium oxide, however, is estimated at $4.20/kg-U ($1.91/lb-U). Hence, with the additional expense of converting the uranium hexafluoride into a suitable uranium oxide aggregate, the cost of the radiation shielding concrete admixture is estimated at about $6.40/kg-U, which is expensive.
In view of the above, it is readily apparent that it would be a significant advancement in the art to provide a cost effective process for converting uranium hexafluoride directly into a stable aggregate for addition to a concrete admixture. More specifically, it would be a significant advancement to provide a one-step process for converting uranium hexafluoride into a suitable aggregate for addition to a concrete admixture.